1. Field of the Invention
This invention pertains to thermal barriers, and more particularly to intumescent thermal barriers and in even greater particularity to foaming intumescent thermal barriers.
2. Description of the Prior Art
Thermal barriers in the form of intumescent paints or tape have been used for protecting coated or taped objects from exposure to high temperatures. Hackh's Chemical Dictionary defines intumescence as the swelling up of a material upon heating. When subjected to heat, intumescent paints begin to expand and form a light carbonatious char of significant thickness. This char layer adhers to the coated objects and expands to many times the thickness of the original coating. Some intumescents expand to 10 or 20 times their original volume. Such intumescents are referred to as foaming intumescents. The char layer is a thermal insulator and thus effectively protects the substrate from heat, and further serves as an oxygen barrier.
U.S. Pat. No. 3,934,066 to Robert Murch describes a variety of intumescent laminate systems for application to combustible or heat sensitive substrates. Generally, intumescent paints, or laminates such as intumescent tapes, expand in the presence of heat to form a thick fire resistant layer having little, if any, structural strength. In an actual fire, the intumescent coating forms a light, heat resisting layer which is easily damaged, for example, by water from a fire hose or from an automatic sprinkler.
In a practical fire fighting strategy the first objective is to knock down the flames, not to bring the burning material instantaneously to room temperature. Hence, the same stream of water which knocks the flames down also removes the intumescent coating from the protected object, and since high temperature still exists in the immediate vicinity, the object will nearly always be reset on fire. Also, the structure to which the intumescent coating is adhesively attached will significantly deform or buckle due to thermal expansion and fire induced thermal stress. The difference in coefficients of thermal expansion between the substrate structure and the intumescent coating combined with its low structural strength results in the coating cracking and separating from the substrate. The coating is then removed from the substrate by gravity, wind, water from a hose, or fire induced connection shears. For these reasons, intumescent coatings standing by themselves have not been practical for use where the protective coating may be damaged prior to use. Mechanical wear, scratches and abrasions can also destroy the effectiveness of a coating because, if the fire can penetrate at a scratch to the substrate, the thermal barrier is bypassed and rendered ineffective. Also, all practical organic coatings are sources of unacceptable smoke production and toxic gases.
Aircraft which are potentially susceptible to exposure in an aircraft carrier deck fuel fire require thermal barriers beneath the cockpit area to prevent flame and toxic gas infiltration of the cockpit. Thermal barriers applied to the exterior of such aircraft are impractical because of the maintenance problems they create, the adverse flight environment such barriers would be exposed to, and their detrimental effects upon aircraft performance.